Ceramic powders

R. J. Pugh, Dispersion and Stability of Ceramic Powders, in Surface and Colloid Chemistry in Advanced Ceramics Processing, Marcel Dekker, New York, 1994, Chapter 4. 

Initially in ceramic powder processing, particle surfaces are created tliat increase tlie surface energy of tlie system. During shape fomiing, surface/interface energy and interiiarticle forces are controlled witli surface active additives. 

Ultimately, the surface energy is used to produce a cohesive body during sintering. As such, surface energy, which is also referred to as surface tension, y, is obviously very important in ceramic powder processing. Surface tension causes liquids to fonn spherical drops, and allows solids to preferentially adsorb atoms to lower tire free energy of tire system. Also, surface tension creates pressure differences and chemical potential differences across curved surfaces tlrat cause matter to move. 

Johnson D W Jr 1987 Innovations in oeramio powder preparation Ceramic Powder Science, Advances in Ceramics vol 21, ed G L Messing ef a/(Westerville, OFI The Amerioan Ceramio Sooiety) pp 3-19... 

Ganguli D and Chatteqee M 1997 Ceramic Powder Preparation Handbook NorweW, MA Kluwer)... 

ADVANCEDCERAMICS - ELECTRONIC CERAMCS] (Vol 1) Electronic ceramic powders... 

Powder Preparation. The goal in powder preparation is to achieve a ceramic powder which yields a product satisfying specified performance standards. Examples of the most important powder preparation methods for electronic ceramics include mixing/calcination, coprecipitation from solvents, hydrothermal processing, and metal organic decomposition. The trend in powder synthesis is toward powders having particle sizes less than 1 p.m and Httie or no hard agglomerates for enhanced reactivity and uniformity. Examples of the four basic methods are presented in Table 2 for the preparation of BaTiO powder. Reviews of these synthesis techniques can be found in the Hterature (2,5). 

Table 2. Methods Used to Prepare BaTiO Electronic Ceramic Powders...

Uniaxial pressing is the method most widely used to impart shape to ceramic powders (24). Binders, lubricants, and other additives are often incorporated into ceramic powders prior to pressing to provide strength and assist in particle compaction (25). Simple geometries such as rectangular... 

K. Osseo-Asare, F. J. Arriagada, and J. H. Adair, "Solubility Relationships in the Coprecipitation Synthesis of Barium Titanate Heterogeneous Equihbria in the Ba—Ti—C2O4—H2O System," in G. L. Messing, E. R. Fuller, Jr., and Hans Hausin, eds.. Ceramic Powder Science Vol. 2,1987, pp. 47-53. 

J. H. Adair, A. J. Roese, and L. G. McCoy, "Particle Size Analysis of Ceramic Powders," in K. M. Nair, ed.,Hdrances in Ceramics, Vol. 2, The American Ceramic Society, Columbus, Ohio, 1984. 

S. Natansohn and V. Sarin in H. Hausner, G. Messing, and S. Hirano, eds.. Ceramic Powder Processing Science, Deutsche Keramische GeseUschaft e.V. Knfn, Germany, 1989, p. 433. 

The most significant commercial product is barium titanate, BaTiO, used to produce the ceramic capacitors found in almost all electronic products. As electronic circuitry has been rniniaturized, demand has increased for capacitors that can store a high amount of charge in a relatively small volume. This demand led to the development of highly efficient multilayer ceramic capacitors. In these devices, several layers of ceramic, from 25—50 ]lni in thickness, are separated by even thinner layers of electrode metal. Each layer must be dense, free of pin-holes and flaws, and ideally consist of several uniform grains of fired ceramic. Manufacturers are trying to reduce the layer thickness to 10—12 ]lni. Conventionally prepared ceramic powders cannot meet the rigorous demands of these appHcations, therefore an emphasis has been placed on production of advanced powders by hydrothermal synthesis and other methods. 

Several manufacturers of ceramic powders are involved in commercializa tion of hydrothermaHy derived powders. In the United States, Cabot (Boyertown, Peimsylvania) has built a small manufacturing plant and is supplying materials to capacitor manufacturers. Other manufacturers include Sakai Chemical and Euji Titanium in Japan. Sakai Chemical is reportedly producing 1 t/d in its demonstration plant. A comparison of the characteristics of commercially available powders is given in Table 2. 

Binders in Ceramics, Powder Metallurgy, and Water-Based Coatings of Fluorescent Lamps. In coatings and ceramics appHcations, the suspension rheology needs to be modified to obtain a uniform dispersion of fine particles in the finished product. When PEO is used as a binder in aqueous suspensions, it is possible to remove PEO completely in less than 5 min by baking at temperatures of 400°C. This property has been successfully commercialized in several ceramic appHcations, in powder metallurgy, and in water-based coatings of fluorescent lamps (164—168). 

R. Heistand 11 and co-workers, ia L. Hencli and D. Ulrich, eds.. Synthesis eA Processing of Suhmicrometer Ceramic Powders, Science of Ceramic Chemical Processing Wiley-Interscience, New York, 1986, pp. 482—495. 

Vapor-Phase Techniques. Vapor-phase powder synthesis teclmiques, including vapor condensation, vapor decomposition, and vapor—vapor, vapor—Hquid, and vapor—soHd reactions, employ reactive vapors or gases to produce high purity, ultrafine, reactive ceramic powders. Many nonoxide powders, eg, nitrides and carbides, for advanced ceramics are prepared by vapor-phase synthesis. 

Vapor—vapor reactions (14,16,17) are responsible for the majority of ceramic powders produced by vapor-phase synthesis. This process iavolves heating two or more vapor species which react to form the desired product powder. Reactant gases can be heated ia a resistance furnace, ia a glow discharge plasma at reduced pressure, or by a laser beam. Titania [13463-67-7] Ti02, siUca, siUcon carbide, and siUcon nitride, Si N, are among some of the technologically important ceramic powders produced by vapor—vapor reactions. 

Vapor—sohd reactions (13—17) are also commonly used ia the synthesis of specialty ceramic powders. Carbothermic reduction of oxides, ia which carbon (qv) black mixed with the appropriate reactant oxide is heated ia nitrogen or an iaert atmosphere, is a popular means of produciag commercial SiC, Si N, aluminum nitride [24304-00-3], AIN, and sialon, ie, siUcon aluminum oxynitride, powders. 

Forming additives or processing aids (2,33—37) are commonly used to render ceramic powders more processible. Binders and plasticizers (qv) are typically added to improve or aid dry powder and plastic forming, whereas deflocculants, surfactants (qv), and antifoams are commonly used in slurry processing. 

Binder selection depends on the ceramic powder, the size of the part, how it is formed, and the green density and strength requited. Binder concentration is deterrnined by these variables and the particle size, size distribution, and surface area of the ceramic powder. Three percent binder, based on dry weight, generally works for dry pressing and extmsion. 

Oleic acid is a good deflocculant for oxide ceramic powders in nonpolar Hquids, where a stable dispersion is created primarily by steric stabilization. Tartaric acid, benzoic acid, stearic acid, and trichloroacetic acid are also deflocculants for oxide powders in nonpolar Hquids. 

Ceramic forming iavolves consoHdation and mol ding of ceramic powders to produce a cohesive body of the desired size and shape. Ceramic forming operations (38,40—66) are conducted with dry powders, plastic bodies, pastes, and slurries. 

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